There is a plug (and it's mate) underneath the dash on the driver's side that when plugged in, makes my check engine light flash (fast flash.) What the heck does this thing do? It's a two-wire green plug, and I recently noticed it was unplugged. Very possible that it's always been like this. I don't get it. :confused:

....just leave it alone----un-plugged.....it is for things that ya don't need ta mess with unless ya need to....it puts the car into 'test mode'....do NOT drive around with it pluged in

Good to know. As soon as the CE light came on, I knew something was fishy and put things back to status quo. I'm just kind of surprised that it's just sitting there, and not tucked someplace where it wouldn't be flopping around.

Me != stupid
Me = curious

Yep this has been asked and answered a billion times. The green connectors under the dash are for PDI and diagnostic use only, you should leave them alone and unplugged. If you ever get an AP you will use those plugs to flash the ECU, that's about the only time you'd want to plug them in.

We could tell you....but then we would have to kill you and then Mulder would have to spend countless hours investigating your mysterious dissappearance only to reveal that some pyscho from the internet killed you and is wearing your skin like a coat....TMI?